If you go to keepvid.com then enter the youtube video url to download the video and click download it gives you option to rip the video. I usually download the highest quality one which is at the top, it is labelled MAX. But then also at the bottom you will also see an option for "» Download SRT « - Subtitles"

If you do this it will download the .srt file. Once this is downloaded, rename the file (keeping.srt at the end) to the exact name of the video, and you play it through VLC player, it will play the subtites also.